In a typical printing or duplicating (e.g., photocopying) device, individual sheets of blank paper are fed from a paper stack (e.g., a paper supply tray) into the area in which the printing or duplicating takes place. Frequently, such a feed device utilizes a pick roller to accomplish that function.
With paper picking devices there typically exists a critical normal force relationship between the pick roller and the paper stack. Too much normal force (i.e., the force which holds the roller onto the paper stack) will result in multi-feeds and too little normal force will result in feed failures. In conventional devices, either the paper stack or the pick roll is spring loaded against the other to provide normal force for picking. Even with extensive tuning of this force, the result is usually a very narrow range of media that will run reliably on the device. These systems are critically affected by a variety of media characteristics: for example, density, net weight, stiffness, and smoothness of the media surface. When the paper stack is spring loaded, density or net weight of the media can vary the resultant normal force. When the pick roller is spring loaded against the paper, the problem with counterbalancing the weight is eliminated but the media stiffness and media surface smoothness are still problems. Presently, the common way to deal with these problems is through a force adjustment mechanism that requires operator intervention when switching from media to media. U.S. Pat. No. 3,306,491, Eisner, et al., issued Feb. 28, 1967, shows a driver roller on a pivoted gear train; there is no mention of autocompensation. U.S. Pat. 4,934,686, Ono, et al., issued Jun. 19, 1990, shows a drive roller on a pivoted arm but not with autocompensation from a gear train.
Autocompensating paper feeders address these issues. In these devices, the pick roller and the paper stack are not spring loaded against one another. The pick roll is mounted on a rotating swing arm. The roll rests on the paper stack. When the pick roll drive is initiated through a gear located on the pivot shaft with the swing arm and counter-rotating to the direction of feed, a torque is applied to the swing arm through the downstream gear train which rotates the swing arm and pick roll into the paper stack. The normal force generated is dictated by the buckling resistance of the media being picked. In theory, the normal force generated is no more than is required to buckle a single sheet of media plus the friction resistance between the first and second sheets. As soon as the sheet has moved, the normal force automatically relaxes. The beauty of this picking device is that it never delivers more normal force than what is required to feed a single sheet of media, and its picking power exceeds any foreseeable requirement. Such a device is taught in U.S. Pat. No. 5,527,026, Padget, et al., issued Jun. 18, 1996. Since these paper feeders develop exactly the amount of force necessary to move each sheet of paper, they require the use of rollers which have very high coefficients of friction (about 1.7 or higher).
To be effective in this sort of environment, the materials used to make the pick (feed) rollers must satisfy a long list of stringent and sometimes inconsistent performance criteria. Specifically, the elastomer used must meet the following requirements:
High Friction--the material must be high friction and maintain that high friction over the lifetime of the machine, typically 250,000 sheets of paper fed.
Non-Glazing--the material must be non-glazing over its entire lifetime. That is, it must not attract paper (or other) dust so that the dust builds up on the surface of the roller and reduces the high friction.
Wear Resistant--the material must be such that it does not wear down or become smooth, over the course of its life, thereby causing increased chances of misfeeds.
Non-Blooming--the elastomer material must not bleed powders to its surface ("blooming") since this will reduce the high friction. Such bleeding occurs when powder ingredients are present in the elastomer above their solubility level.
Non-Staining--the material must not stain the paper as the roller rests on it. That is, ingredients that stain must not bleed out of the elastomer material.
Non-Marking--the material must not leave marks when it rubs against the paper during feeding.
Ozone Resistant--ozone resistance (that is, resistance to surface cracking due to atmospheric ozone) is important since ozone cracks accelerate the surface wear of the roll.
Oxidation Resistant--oxidation resistance is necessary since oxidation of the material causes it to become sticky, load up with paper dust, and lose its high friction.
Made From Commonly Available Ingredients--the elastomer ingredients must be commonly available so that supply will be constant during the entire time the machine is manufactured.
Low Cost--cost is a consideration since the machine must be competitively priced.
It has now been found that specific rubber compositions, when used in paper-moving pick rollers, meet all of these criteria, and are effective, long-lasting paper feed pick rollers, particularly in autocompensating feed devices. It has also been found that the relatively soft, high friction rollers of the present invention minimize the number of double or multiple feeds which occur.